Susanne Jock

Identification of Erwinia amylovora by growth morphology on agar containing copper sulfate and by capsule staining with lectin

Erwinia amylovora strains formed yellow colonies on minimal agar medium MM2 containing asparagine and copper sulfate (MM2Cu), in contrast to a white morphology on minimal agar without copper salt. Additionally, the colonies were mucoid to various extents. The yellow color was characteristic for the fire blight pathogen, including strains from raspberry and from other unusual host plants, and was used to establish a novel plating technique for identification of E. amylovora. The new identification method was especially superior to semi-selective media with sucrose when natural levan-deficient strains were assayed. No growth of E. amylovora was observed for the similar medium MM1 containing 2 mM CuSO4, due to its low content of as paragine. Identification by colony morphology on MM2 agar with copper was confirmed by staining the bacterial capsules with FITC-labeled lectin from Abrus precatorious, a compound which has a high affinity for galactose residues, the main sugar in the capsular exopolysaccharide amylovoran of E. amylovora. Other plant-associated bacteria usually did not produce the typical colony morphology of E. amylovora on MM2 agar with copper. Furthermore, those cells were not stained with the Abrus lectin. Capsule staining was also observed for weakly mucoid strains of E. amylovora, but not for strains with mutations affecting amylovoran synthesis. The secretion of fluorescent compounds by Pseudomonas syringae pathovars and even growth of any other bacterial colonies adjacent to E. amylovora could interfere with the formation of typical yellow colonies on MM2Cu, which could be white in case of dense plating. After screening for white colonies on LB agar, E. amylovora was identified in extracts from Cotoneaster leaves and in bark from apple trees with fire blight symptoms by its yellow growth pattern on MM2Cu agar and by capsule staining. The proposed selective medium gives a clear signal, is easy to prepare, does not contain dyes or any compounds toxic to humans, and can also detect E. amylovora strains deficient in levan synthesis.

Molecular comparison of pathogenic bacteria from pear trees in Japan and the fire blight pathogen Erwinia amylovora.

Several strains of the genus Erwinia, which were isolated in Japan from pear trees with necrotic symptoms that resembled fire blight, and tentatively identified as Erwinia amylovora, were reinvestigated for their relationship to the fire blight pathogen. These isolates produced ooze on slices of immature pears and were mucoid on MM2Cu agar plates, but did not synthesize levan and did not give the expected PCR signals with several primer pairs specific for Erwinia amylovora. The isolates tested positive with PCR primers designed to detect the novel pear pathogen Erwinia pyrifoliae, which was isolated from Nashi pear trees in South Korea. The nucleotide sequence analysis of a DNA fragment preceding the gene cluster for exopolysaccharide synthesis revealed a closer relationship to Erwinia pyrifoliae than to Erwinia amylovora. Plasmid profiles, protein patterns and genomic DNA analysed by PFGE after XbaI and SpeI digestion were different than Erwinia amylovora. Experiments with strains of Erwinia amylovora isolated from raspberry (Rubus sp.), Erwinia mallotivora and Enterobacter pyrinus also did not reveal a relationship between these bacteria and the Japanese Erwinia strains. The latter are not identical to Erwinia pyrifoliae, but possess many similar features to this pathogen that causes Asian pear blight. It is concluded that pathogenic bacteria isolated in Japan from pear trees with symptoms resembling fire blight are possibly different from Erwinia amylovora.

Molecular Detection and Differentiation of Erwinia pyrifoliae and Host Range Analysis of the Asian Pear Pathogen

The recently described pathogen Erwinia pyrifoliae, isolated from Nashi pear fruit trees in Korea, resembles the fire blight pathogen Erwinia amylovora in some of its properties. The two pathogens were classified into different species by DNA hybridization kinetics and microbiological criteria. From the nucleotide sequences of the 16S rRNA and the internal transcribed spacer (ITS) region as well as extracellular polysaccharide (EPS)-encoding genes, polymerase chain reaction (PCR) primers were designed that specifically detect E. pyrifoliae but not the fire blight pathogen Erwinia amylovora, and these primers were also applied to identify E. pyrifoliae in necrotic plant material. The genomes of several strains were digested with the restriction enzyme SpeI, and the DNA fragments were analyzed by pulsed-field gel electrophoresis (PFGE). Three groups of patterns could be distinguished for the isolated E. pyrifoliae strains, all different from various E. amylovora strains, which produce a relatively homogeneous PFGE pattern after SpeI digests. Typical fire blight host plants were assayed in a growth chamber or an experimental field for their susceptibility to E. pyrifoliae. A strong preference was found for pear varieties, whereas apple, cotoneaster, hawthorn, or raspberry rarely produced necrotic symptoms. E. pyrifoliae was readily detected in samples from pear orchards in South Korea during 1995 to 1998; however, the Asian pear pathogen was not recovered in necrotic plant tissue from 1999 and 2000.

Identification of Pantoea stewartii subsp. stewartii by PCR and strain differentiation by PFGE.

Stewarts bacterial wilt and leaf blight of sweet corn and maize is caused by Pantoea stewartii subsp. stewartii. This bacterium can be seed transmitted at a low frequency, so it is subject to quarantine restrictions by many countries. To develop a polymerase chain reaction assay for the identification of this pathogen from field samples and for use in seed health tests, four primer pairs were tested. These were selected from the sequences of hrpS, cpsDE, and the 16S rRNA intergenic transcribed spacer (ITS) region. Under optimal reaction conditions, about 20 and 200 cells of P. stewartii could be detected in pure cultures and leaf lesions, respectively. Other plant-associated enteric bacteria (e.g., P. agglomerans pv. herbicola, P. ananas, Erwinia amylovora, and E. carotovora) either did not produce amplicons or they were not the correct size for P. stewartii. To test further for possible false positives, 29 yellow-pigmented bacteria, mainly other Pantoea spp., were isolated from lesions on old corn leaves and assayed with the ITS primer sets. Except for weak, variable reactions with three P. ananas strains, the bacteria did not test positive. Pulsed field gel electrophoresis (PFGE) was evaluated as an additional test to confirm the identity of P. stewartii. After digestion with SpeI and XbaI, P. stewartii strains could be easily distinguished from related Erwinia and Pantoea spp. and each other.

Hypersensitive response and acyl‐homoserine lactone production of the fire blight antagonists Erwinia tasmaniensis and Erwinia billingiae

Fire blight caused by the Gram‐negative bacterium Erwinia amylovora can be controlled by antagonistic microorganisms. We characterized epiphytic bacteria isolated from healthy apple and pear trees in Australia, named Erwinia tasmaniensis, and the epiphytic bacterium Erwinia billingiae from England for physiological properties, interaction with plants and interference with growth of E. amylovora. They reduced symptom formation by the fire blight pathogen on immature pears and the colonization of apple flowers. In contrast to E. billingiae, E. tasmaniensis strains induced a hypersensitive response in tobacco leaves and synthesized levan in the presence of sucrose. With consensus primers deduced from lsc as well as hrpL, hrcC and hrcR of the hrp region of E. amylovora and of related bacteria, these genes were successfully amplified from E. tasmaniensis DNA and alignment of the encoded proteins to other Erwinia species supported a role for environmental fitness of the epiphytic bacterium. Unlike E. tasmaniensis, the epiphytic bacterium E. billingiae produced an acyl‐homoserine lactone for bacterial cell‐to‐cell communication. Their competition with the growth of E. amylovora may be involved in controlling fire blight.

Expression of Immunogenic Puumala Virus Nucleocapsid Protein in Transgenic Tobacco and Potato Plants

Transgenic plants, expressing recombinant proteins, are suitable alternatives for the production of relevant immunogens. In the present study, the expression of Puumala virus nucleocapsid protein in tobacco and potato plants (Nicotiana tabacum and Solanum tuberosum) and its immunogenicity was investigated. After infection of leaf discs of SR1 tobacco and tuber discs of potato cv. “Desiree” with the Agrobacterium strain LBA4404 (pAL4404, pBinAR-PUU-S) containing the 1302 bp cDNA sequence of S-RNA segment of a Puumala virus, transgenic tobacco and potato plants expressed the Puumala virus nucleocapsid protein under control of the cauliflower 35S promoter. The recombinant proteins were found to be identical to the authentic Puumala virus nucleocapsid protein as analyzed by immunoblotting. Expression of the nucleocapsid protein was investigated over four plant generations (P to F4) and found to be stable (1 ng/3 μg dried leaf tissue). Transgenic tobacco plants were smaller compared to controls. The transformed potato plants were morphologically similar to control plants and produced tubers as the control potatoes. The S-antigen was expressed at a level of 1 ng protein/5 μg and 1 ng protein/4 μg dried leaf and root tissues, respectively, and remained stable in the first generation of vegetatively propagated potato plants. The immunogenicity of the Puumala virus nucleocapsid protein expressed in Nicotiana tabacum and Solanum tuberosum was investigated in New Zealand white rabbits. They were immunized with leaf extracts from transgenic tobacco and potato plants, and the serum recognized Puumala virus nucleocapsid protein. Transgenic plants expressing hantaviral proteins can thus be used for the development of cost-effective diagnostic systems and for alternative vaccination strategies.

Molecular Characterization of Natural Erwinia pyrifoliae Strains Deficient in Hypersensitive Response

ABSTRACT From necrotic tissue of a Nashi pear tree, 24 Erwinia pyrifoliae strains, found to be identical by pulsed-field gel electrophoresis analysis, were isolated. Thirteen strains were not virulent on immature pears and did not induce a hypersensitive response in tobacco leaves. The defective gene hrpL was complemented with intact genes from E. pyrifoliae and Erwinia amylovora.

Characterization of Erwinia amylovora strains from Croatia

Erwinia amylovora is the causative agent of fire blight, a destructive disease of rosaceous plants. The European population can be divided into several subtypes according to differences in restriction fragment length polymorphism of the XbaI genomic DNA digest analysed with pulsed-field gel electrophoresis. This technique was also used to determine the genetic relatedness of six Croatian isolates to the E. amylovora types found in the countries surrounding Croatia. The isolates belong to the Pt2 pattern type that is characteristic of the East Mediterranean basin. All tested isolates gave essentially the same total cell protein pattern in SDS-polyacrylamide gel electrophoresis. The number of short-sequence DNA repeats in plasmid pEA29 of six isolates was determined by PCR assays and ranged from four to seven. The isolates examined showed high pathogenicity in immature pear fruits. Differences were also revealed in microbiological assays such as amylovoran synthesis, levan formation, siderophore production and colour on coliform medium.

Molecular analyses of Erwinia amylovora strains isolated in Russia, Poland, Slovenia and Austria describing further spread of fire blight in Europe

Fire blight, a bacteriosis of apple and pear, was assayed with molecular tools to associate its origin in Russia, Slovenia and south-eastern Austria with neighboring countries. The identification of all investigated strains was confirmed by MALDI-TOF mass spectroscopy except one. Independent isolation was verified by the level of amylovoran synthesis and by the number of short sequence DNA repeats in plasmid pEA29. DNA of gently lysed E. amylovora strains from Russia, Slovenia, Austria, Hungary, Italy, Spain, Croatia, Poland, Central Europe and Iran was treated with restriction enzymes XbaI and SpeI to create typical banding patterns for PFGE analysis. The pattern Pt2 indicated that most Russian E. amylovora strains were related to strains from Turkey and Iran. Strains from Slovenia exhibited patterns Pt3 and Pt2, both present in the neighboring countries. Strains were also probed for the recently described plasmid pEI70 detected in Pt1 strains from Poland and in Pt3 strains from other countries. The distribution of pattern Pt3 suggests distribution of fire blight from Belgium and the Netherlands to Central Spain and Northern Italy and then north to Carinthia. The PFGE patterns indicate that trade of plants may have introduced fire blight into southern parts of Europe proceeded by sequential spread.

Unifying bacteria from decaying wood with various ubiquitous Gibbsiella species as G. acetica sp. nov. based on nucleotide sequence similarities and their acetic acid secretion

Bacteria were isolated from necrotic apple and pear tree tissue and from dead wood in Germany and Austria as well as from pear tree exudate in China. They were selected for growth at 37 °C, screened for levan production and then characterized as Gram-negative, facultatively anaerobic rods. Nucleotide sequences from 16S rRNA genes, the housekeeping genes dnaJ, gyrB, recA and rpoB alignments, BLAST searches and phenotypic data confirmed by MALDI-TOF analysis showed that these bacteria belong to the genus Gibbsiella and resembled strains isolated from diseased oaks in Britain and Spain. Gibbsiella-specific PCR primers were designed from the proline isomerase and the levansucrase genes. Acid secretion was investigated by screening for halo formation on calcium carbonate agar and the compound identified by NMR as acetic acid. Its production by Gibbsiella spp. strains was also determined in culture supernatants by GC/MS analysis after derivatization with pentafluorobenzyl bromide. Some strains were differentiated by the PFGE patterns of SpeI digests and by sequence analyses of the lsc and the ppiD genes, and the Chinese Gibbsiella strain was most divergent. The newly investigated bacteria as well as Gibbsiella querinecans, Gibbsiella dentisursi and Gibbsiella papilionis, isolated in Britain, Spain, Korea and Japan, are taxonomically related Enterobacteriaceae, tolerate and secrete acetic acid. We therefore propose to unify them in the species Gibbsiella acetica sp. nov.